This invention relates, in general, to masks used in making semiconductor devices and, more particularly, to making phase-shift masks.
At present, in the manufacturing of semiconductor devices small features or small geometric patterns are created by using conventional optical photolithography. Typically, optical photolithography is achieved by projecting or by transmitting light through a pattern made of optically opaque areas and optically clear areas on a mask. The optically opaque areas of the pattern block the light, thereby casting shadows and creating dark areas, while the optically clear areas allow the light to pass, thereby creating light areas. Once the light areas and dark areas are formed, they are projected onto and through a lens and subsequently onto a photosensitive layer on a semiconductor substrate. Typically, the lens reduces the light and dark areas or pattern by either 4.times., 5.times., or 10.times.. Additionally, all dimensions hereinafter are mask dimensions for a 5.times. reduction lithographic tool. However, it should be understood that dimensions can be scaled for use with other reduction tools. Projecting light areas and dark areas on the photosensitive layer results in portions of the photosensitive layer being exposed, while other portions of the photosensitive layer will be unexposed.
However, because of increased semiconductor device complexity, which results in increased pattern complexity, increased resolution demands, and increased pattern packing density on the mask, distance between any two opaque areas has decreased. By decreasing the distances between the opaque areas, small apertures are formed which diffract the light that passes through the apertures. The diffracted light results in effects that tend to spread or to bend the light as it passes so that the space between the two opaque areas is not resolved, therefore making diffraction a severe limiting factor for conventional optical photolithography.
A method for dealing with diffraction effects in conventional optical photolithography is achieved by using a chromeless phase-shift mask, which replaces the previously discussed mask. Generally, with light being thought of as a wave, phase shifting with a chromeless phase-shift mask is achieved by effecting a change in timing or by effecting a shift in waveform of a regular sinusoidal pattern of light waves that propagate through a transparent material. Typically, phase shifting is achieved by passing light through areas of a transparent material of either differing thicknesses or through materials with different refractive indexes, thereby changing the phase or the period pattern of the light wave.
Chromeless phase-shift masks reduce diffraction effects by combining both phase shifted light and nonphase shifted light so that constructive and destructive interference takes place. Generally, a summation of constructive and destructive interference of phase-shift masks results in improved resolution and in improved depth of focus of a projected image of an optical system. However, chromeless phase-shift masks have problems when the summation of the constructive and destructive light waves result in inappropriate bright spots or hot spots in a projected pattern. The inappropriate hot spots or bright spots unintentionally exposes portions of the photosensitive layer, thereby creating an incorrect pattern on the photosensitive layer. These hot spots or bright spots typically occur when two phase-shift elements meet. Typically, the phase-shift elements form a right angle or meet an edge of a large feature; however, the bright spots or hot spots occur at varying degrees of intensity in response to angle changes. Accordingly, it is desirable to make a chromeless phase-shift mask that does not produce inappropriate hot spots or bright spots, that can project angles effectively, and that allows for ease in manufacturing, while still achieving the benefits of using a chromeless phase-shift mask.